Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What's the implication of calulationing the apparent angular sizes of a galaxy

  1. May 22, 2008 #1
    Now suppose for simplicity that the galaxy,and that its physical diameter was w at the time it emitted the light.find the apparent angular size of the galaxy as it would be observed from earth today.
    the answer given by Mr.Guth seems imply that the galaxy itself is also expanding and point out that objects seen at very large redshifts are seen in a smaller universe so they look unusually large.And he (maybe the teaching assistant)compared the result with the one that only take recession of the galaxy into consideration.
    But as far as I know galaxy don't expand with the sapce-time background.I'm quite confused.
  2. jcsd
  3. May 22, 2008 #2


    User Avatar
    Science Advisor

    Have you heard of angular diameter distance and how it is calculated in cosmology? If you find this is your textbook that should answer your question.
  4. May 23, 2008 #3


    User Avatar
    Science Advisor
    Gold Member

    There is no evidence suggesting factors influencing galactic evolution are invariable over time. The universe was much hotter in the distant past. It is reasonable to suspect this may have had a significant effect on galactic evolution.
  5. May 23, 2008 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Did Wallace's hint---to look up "angular size distance" in your text---work for you or are you still confused?

    If you are still confused, you might be more explicit about your situation. You referred to Mr Guth, is that your teacher or is he the author of the book you are studying from like for example Inflationary Universe by Alan Guth:

    I can't tell exactly what is puzzling you because you don't say enough.

    I can GUESS that what is puzzling you is a kind of wellknown paradox about angular size. Somebody---like your textbook author or your teaching assistant---may have been trying to explain that and gotten you confused.

    It does not have to do with galaxies changing their actual physical size. If all galaxies were forever and always the exact same diameter you would still see a curious effect due to the expansion history.

    Suppose that all galaxies are and have always been exactly 100,000 LY in diameter.

    Then you intuitively expect that the ones that are farther away will "look smaller" in the sense of making a smaller angle in the sky. Like a distant car on the road looks smaller than a nearby car----makes a smaller angle in your field of vison.

    But because the universe has expanded it doesn't work like what you intuitively expect.
    what we see as the angular size of a galaxy is the angular size that it HAD when it emitted the light
    which could be larger than you expect (if it emitted the light a long time ago when it was much closer).

    Here is an example. The CMB radiation that people love to make temperature maps of these days is coming to us from hydrogen gas that was once (at the time it emitted the light) 40 million LY from here. Imagine an hot blob of that gas that was diameter 1 million LY. So across the blob makes an angle of 1/40 of a radian. The lightrays coming to us span an angle of 1/40 radian.

    Assume the blob stays the same diameter (1 million LY) as it might if it condenses into a gravitationally bound cluster. Stable objects like clusters and galaxies don't participate in the general expansion.

    So now that hydrogen is 44 billion LY away from here, because of the 1100-fold expansion which has occurred since the CMB light was emitted. That expansion is why the light took so long to get here, almost the whole 13-some billion years.

    Intuitively you think that the angular diameter should be 1/44 MILLIradians. Because the object is one million LY diameter and it is now 44 billion LY away. But the measured angular diameter is not that small. It is actually 1100-fold larger than you expect, namely 1/40 of a radian.

    The example illustrates a general fact that stuff at cosmological distances looks larger (angular size) than you'd expect.

    That MAY be what the book or the teacher was trying to get across. If it was something else and you are still puzzled, keep asking and make it more clear what the problem is.
    Last edited by a moderator: Apr 23, 2017
  6. May 24, 2008 #5
    OMG,it is so so so stupid a question.........thank you all for your waste time explaining.
    I was doing the problem sets of Mr.Guth downloaded from internet.And I got the wrong idea of interpreating the answer as the angular size of the galaxy emitted today which will be observed in the future.From this viewpoint I got to the wrong conclusion that the answer itself implies an expanding galaxy.
  7. May 24, 2008 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    It seems like a smart thing to do, to find problem sets for a cosmology course on the internet and go thru and work the problems.
    How about giving us the link to the problem sets, so we know what you are talking about?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: What's the implication of calulationing the apparent angular sizes of a galaxy